MYB98 Positively Regulates a Battery of Synergid-Expressed Genes Encoding Filiform Apparatus–Localized Proteins

Abstract

The synergid cells within the female gametophyte are essential for reproduction in angiosperms. MYB98 encodes an R2R3-MYB protein required for pollen tube guidance and filiform apparatus formation by the synergid cells. To test the predicted function of MYB98 as a transcriptional regulator, we determined its subcellular localization and examined its DNA binding properties. We show that MYB98 binds to a specific DNA sequence (TAAC) and that a MYB98-green fluorescent protein fusion protein localizes to the nucleus, consistent with a role in transcriptional regulation. To identify genes regulated by MYB98, we tested previously identified synergid-expressed genes for reduced expression in myb98 female gametophytes and identified 16 such genes. We dissected the promoter of one of the downstream genes, DD11, and show that it contains a MYB98 binding site required for synergid expression, suggesting that DD11 is regulated directly by MYB98. To gain insight into the functions of the downstream genes, we chose five genes and determined the subcellular localization of the encoded proteins. We show that these five proteins are secreted into the filiform apparatus, suggesting that they play a role in either the formation or the function of this unique structure. Together, these data suggest that MYB98 functions as a transcriptional regulator in the synergid cells and activates the expression of genes required for pollen tube guidance and filiform apparatus formation.

Whole Genome Analysis of Gene Expression Reveals Coordinated Activation of Signaling and Metabolic Pathways during Pollen-Pistil Interactions in Arabidopsis

“…Using the parameters outlined in "Materials and Methods" and further comparison with publicly available data sets (Zimmermann et al, 2004;Tung et al, 2005;Yu et al, 2005;Steffen et al, 2007;Borges et al, 2008;Wang et al, 2008;Qin et al, 2009;Wuest et al, 2010), a total of 42 genes were considered enriched or preferentially expressed in ovules (Supplemental File S1). As anticipated, 27 out of 42 transcripts preferentially expressed in ovules and 96 out of 1,275 ovule-enriched transcripts were previously reported to be expressed specifically in the embryo sac or enriched in female gametophytic cells (Supplemental File S1; Yu et al, 2005;Johnston et al, 2007;Punwani et al, 2007;Steffen et al, 2007;Wuest et al, 2010). Ovule-expressed transcripts likely include plausible candidates for shortrange pollen tube attractants expressed in the embryo sac (Higashiyama et al, 2001(Higashiyama et al, , 2003Chen et al, 2007;Okuda et al, 2009) but also of other modulators expressed in the surrounding sporophytic tissues that could function as long-range pollen tube attractants.…”

Truncation of a Protein Disulfide Isomerase, PDIL2-1, Delays Embryo Sac Maturation and Disrupts Pollen Tube Guidance inArabidopsis thaliana

“…The gametophytic phase, after the tube appears on the placental surface in the ovary, can be further divided into funicular guidance (Shimizu and Okada, 2000) and micropylar guidance (Higashiyama et al, 1998(Higashiyama et al, , 2001. Sporophytic tissues play some role in funicular guidance (Baker et al, 1997;Palanivelu et al, 2003), while the synergid cells (Higashiyama et al, 2001;Punwani et al, 2007) and central cell (Chen et al, 2007) are required for micropylar guidance.…”

Pollen Tube Guidance by Attractant Molecules: LUREs

“…Many CRPs of Arabidopsis are also suggested to be expressed in the synergid cell, as CRPs are downregulated in the ovule of a knockout mutant of the MYB98 gene (Jones-Rhoades et al, 2007;Punwani et al, 2007). Using GFP fusion, some CRPs were shown to be secreted in the direction of the filiform apparatus (Punwani et al, 2007).…”

“…Using GFP fusion, some CRPs were shown to be secreted in the direction of the filiform apparatus (Punwani et al, 2007). In EST analysis of the isolated egg cell (Cordts et al, 2001) and embryo sac (Yang et al, 2006) of maize, various CRPs were shown to be expressed specifically in the female gametophyte.…”